This invention relates to a method and system for attaching semiconductor dice to a leadframe, and more particularly to such a method and system wherein an instant curing adhesive, such as cyanoacrylate is utilized to bond the dice to the leadframe.
A conventional semiconductor package is formed by attaching several semiconductor dice to a metal leadframe. The leadframe comprises a metal strip that supports the dice for packaging and provides the leads in the completed packages. One type of leadframe includes several mounting paddles for attaching each die to the leadframe. Each paddle has a peripheral configuration which corresponds to the peripheral configuration of a semiconductor die. The leadframe also includes lead fingers that can be wire bonded to the bond pads on the dice. Following attachment and wire bonding of the dice to the leadframe, the dice and mounting paddles can be encapsulated in plastic, and the lead fingers trimmed to form the package leads.
Another type of leadframe is known as a lead-on-chip (LOC) leadframe. A LOC leadframe does not include mounting paddles for the dice. With a LOC leadframe, the lead fingers not only form the leads for the completed package, but also support the dice during the encapsulation process. Typical lead-on-chip (LOC) dice include bond pads formed along the center of the die. The dice can be attached directly to die attach areas on the lead fingers, and the bond pads on the dice wire bonded to bonding sites on the lead fingers.
Die attachment to the leadframe is designed to create a strong bond between the dice and the packages, the integrity of which can withstand subsequent processing and use in an electronic product. In addition, the material used for attaching the dice to the leadframe can serve other purposes such as heat transfer, and in some cases, electrical conductivity between the dice and lead fingers.
One conventional method of attaching dice to a leadframe is with polymer adhesives, such as epoxy and polyimide. Polymer adhesives are commercially available as viscous materials which can be deposited on the mounting paddles of the leadframe, such as by dispensing with a syringe, or screen printing. Using a die attach mechanism, each die can be positioned over a die attach area, then pressed onto the leadframe to form a thin adhesive layer between the die and the leadframe. Finally, the leadframe and die can be placed in an oven, to cure the adhesive at elevated temperatures in an inert atmosphere. Typical curing temperatures are from 150xc2x0 C. to 275xc2x0 C. for thirty minutes to one hour, in a nitrogen atmosphere.
For attaching LOC dice to a LOC leadframe, a double-sided adhesive tape can be used in place of a dispensed adhesive. The adhesive tape can include a thermoset adhesive on both sides, such as an epoxy, acrylic, silicone or polyimide. Such LOC tape is initially attached to the leadframe, and then to the faces of the dice using heat and pressure. Finally, the tape can be cured by subjecting the packages to elevated temperature in an oven.
Each semiconductor die has a thermal budget during processing and packaging, in that each die may only be subjected to a certain amount of heat without inducing thermal stresses. These thermal stresses can lead to loss of physical integrity or processing function in a die. It is generally accepted that the amount of heat to which dice are subjected during processing and packaging should be minimized. Thus, the use of heat curable adhesives for attaching semiconductor dice to leadframes during packaging are problematic. It would be advantageous to be able to attach semiconductor dice to leadframes without heating and introducing thermal stresses on the dice. It would also be advantageous to be able to attach semiconductor dice to leadframes at room temperature and ambient atmosphere.
The present invention is directed to a method and system for attaching semiconductor dice to leadframes, including LOC leadframes, without inducing thermal stresses on the dice. Still further, the present invention is directed to a method and system for attaching semiconductor dice to leadframes, including LOC leadframes, which can be performed in-line at room temperature, and ambient atmosphere with a high throughput.
In accordance with the present invention, an improved method for attaching semiconductor dice to leadframes, an improved semiconductor package, and a system for performing the method are provided. The method, simply stated, comprises: applying an instant curing adhesive to a portion of a leadframe; placing a semiconductor die in contact with the adhesive; and then polymerizing the adhesive at room temperature and ambient atmosphere to form a cured adhesive layer and bond the die to the leadframe.
Suitable instant curing adhesives include cyanoacrylate adhesives, and anaerobic acrylic adhesives. Commercial formulations of these adhesives can be modified to tailor the characteristics of the cured adhesive layer to electronics packaging. These characteristics can include improved mechanical strength, heat conductivity, moisture resistivity, and thermostablity. In addition, depending on the application, the instant curing adhesive can be formulated with an increased electrical conductivity or alternately with an increased dielectric strength.
Following curing of the adhesive, and bonding of the die to the leadframe, the die can be wire bonded to the leadframe and encapsulated in a polymer, and the leadframe trimmed to form a singulated semiconductor package. The semiconductor package comprises a trimmed portion of leadframe, the encapsulated semiconductor die, and the cured adhesive layer.
The method can be performed with a conventional lead frame having mounting paddles, or with a lead-on-chip (LOC) lead frame having lead fingers configured to support the die for packaging. With a conventional leadframe, the adhesive can include conductive fillers selected to improve the electrical conductivity and heat conductivity of the cured adhesive layer. With a LOC leadframe, the adhesive can include non-conductive fillers selected to increase the dielectric strength of the cured adhesive layer and prevent cross-talk between the lead fingers.
The system comprises: a dispensing mechanism for dispensing the instant curing adhesive on the leadframe; and a die attach mechanism for positioning and placing the die in contact with the dispensed adhesive. In the illustrative embodiment, the die attach mechanism comprises a conventional LOC die attacher modified to practice the invention.